The present invention relates to a control circuit for a constant temperature type hot wire air flow meter.
FIG. 1 is a circuit diagram showing one example of the conventional constant temperature type hot wire air flow meter. In the circuit, 1 denotes a hot wire made of platinum or the like. The hot wire 1 is controlled to be a constant temperature by a circuit consisting of operational amplifiers 5, 6, a transistor 7 and resistors 3, 9, 21, 22. Air flow directed at the hot wire 1 results in a change in its resistance value. To cope with the change, the circuit serves to control the resistance value to be constant (the constant resistance value means a constant temperature). At a given time, a voltage is generated across resistor 3. This voltage represents a function of the air flow rate so that it can be used to produce a signal which is a measure of the air flow rate. A resistor 2 is a cold wire used for compensating for the temperature of the air to be measured.
FIG. 2 is an equivalent circuit of an air flow meter shown in FIG. 1. Resistors 1 and 3 in FIG. 2 are arranged in the same manner as those shown in FIG. 1. An amplifier 56 is an equivalent amplifying circuit composed of operational amplifiers 5, 6, resistors 2, 9, 21, 22 and the like. A voltage 256 is a d.c. voltage value defined by the circuit arrangement. This value is an index representing how much the circuit operation shifts out of a linear operation. It can be represented as follows (this value is referred to as a general offset voltage). EQU V.sub.1 =(V.sub.2 +V.sub.OF)G.sub.2 ( 1)
wherein V.sub.2 is a voltage which appears at a branch point between the resistors 1 and 3, V.sub.1 is an output of the amplifier 56, and G is a gain of the amplifier 56.
Further, the voltages V.sub.1 and V.sub.2 have the following relation; EQU V.sub.2 =R.sub.3 /(R.sub.1 +R.sub.3).V.sub.1 ( 2)
From the equations (1) and (2), the voltages V.sub.1 and V.sub.2 can be derived as follows. ##EQU1## As will be understood from the equations (3) and (4), no value of V.sub.1 and V.sub.2 can be provided without a value of V.sub.OF.
That is, the subject circuit provides a general offset voltage V.sub.OF, that is, an element defining the circuit operation in itself. By designing this value properly, the air flow meter reaches an optimum operation.
The value of V.sub.OF is defined by an input offset voltage V.sub.OPF of the operational amplifiers 5 and 6, resistors 24 and 28, a voltage V.sub.CC and the like. To keep the value of V.sub.OF constant, an external circuit composed of resistors 24, 28 and the like may be employed.
Even if the external circuit is used for keeping the value of V.sub.OF constant, this value will inevitably vary because it is subject to various factors such as temperature and power supply voltage level. In particular, since, V.sub.3.sup.OF.sub.2 is likely to be affected by the temperature and since the temperature is likely to vary widely, the value of V.sub.OF under certain conditions may become zero or lower than zero. This results often in a disabling of the circuit shown in FIG. 1 as an air flow meter; bringing it into an oscillating state.
One example of the conventional hot wire air flow meter has been disclosed in the Un-examined Patent Publication JP-A-64-8828 of the Japanese Application Serial No. 62-244096 Hitachi, Ltd., filed on Nov. 30, 1987.